In a display unit in which light emitting elements of a current drive type such as organic EL elements, inorganic EL elements or the like are used as pixels, brightness of each pixel depends on how much a current passing through each light emitting element is. Thus, for attaining even brightness, the display unit is so arranged that voltage conditions of active elements are so controlled that the currents passing through the respective lighting elements are approximately equal.
In case of a display unit of an active matrix type, having an image display section (display screen) in which a number of pixels are arrayed vertically and horizontally, a current is supplied to a light emitting element of each pixel via a current supplying line from a power source, and the current is discharged to a common electrode (ground) via a current discharging line from each light emitting element.
Here, the current supplied to each light emitting element depends on a length of the current supplying line from the power source to the light emitting element or a length of the current discharging line, because of resistance loss on the way.
FIG. 20 illustrates how positions of the light emitting elements relate with current values to be supplied to the light emitting elements, in case the currents are supplied from edge part of the display screen to the light emitting elements via the current supplying lines. Note that in the following the positions of the light emitting elements are indicated by “node numbers” allotted in an ascending order from center part to the edge part. Hereinafter, the current value to be supplied to the light emitting elements are referred to as “node current values”.
FIG. 20 shows that the node current values are smaller as the node numbers descend. In short, the edge part of the display screen, in which the node numbers are greater, is displayed brightly, while the center part of the display screen, in which the node numbers are smaller, is displayed darkly.
In order to reduce a difference between the current values in the edge part and the central part of the display screen, the current supplying lines and the current discharging lines may be made of a highly conductive material having a small relative resistance. However, transparent electrodes such as ITO (Indium Tin Oxide) or the like are generally used in either the current supplying lines or the current discharging lines, so that light from the light emitting elements will be allowed to pass through to outside. The transparent electrodes have a greater relative resistance than the highly conductive metal such as copper, aluminum, and the like. Therefore, there is a limit to the reduction of the difference between the current values.
Moreover, a driving load caused by a plurality of the light emitting elements respectively connected to the current supplying lines is changed according to how many light emitting elements are turned ON. Thus, the node current values may be changed according to how many light emitting elements are turned ON.
For example, discussed below is a case where the currents are supplied from upper edge part and lower edge part of the display screen to each light emitting element, while the central part of the image display section is not turned ON, as shown in FIG. 21. In this case, the light emitting elements that are in a column A are all turned ON. Meanwhile each end part of the light emitting elements that are in a column B is turned ON while each central part thereof is turned OFF. Node current values supplied to the light emitting elements in the columns A and B in this case are shown in FIG. 22. Referring to FIG. 22, the node current values of the columns at a node number are compared with each other. In the area in which both the light emitting elements in column A and column B are turned ON, the node current values in column B are greater than those in column A. Therefore, as shown in FIG. 23, the turned-ON areas above or under the turned-OFF area have a greater brightness than the turned-ON area on the right or on the left of the turned-OFF area.
The following are known as arts of preventing such unevenness in brightness. Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 11-282420 (Date of Release: Oct. 15, 1999), Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 11-327506 (Date of Release: Nov. 26, 1999), and Japanese Publication of Unexamined Patent Application, Tokukaihei, No. 11-344949 (Date of Release: Dec. 14, 1999), disclose arts in which signal data respectively applied on pixels are corrected in accordance with unevenness between light emitting elements in terms of brightness (that is, unevenness between supplying currents).
However, in those arts, a display apparatus additionally requires means for storing each correction value of the light emitting elements. This results in a large circuit size of the display apparatus.
Moreover, Japanese Publication of Unexamined Patent Application, Tokukai, No. 2000-221994 (Date of Release: Aug. 11, 2000) discloses an art in which numbers of light emitting pixels are counted per scanning electrode, and pulse widths of scanning pulse voltages to be applied on the scanning electrodes are set according to the numbers of the light emitting pixels. According to this art, it is possible to reduce unevenness in brightness due to a difference in the numbers of light emitting pixels between adjacent scanning electrodes.
However, in this art, a display apparatus additionally requires means for counting the numbers of the light emitting pixels, and means for changing the pulse widths of the scanning pulse voltages. This results in a large circuit size of the display circuit.